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Definitions

To keep this site somewhat self-contained, we define some technical terms that arise when defining and describing the Cardano blockchain and ledger. This is not meant to be comprehensive and the reader may wish to consult online resources to fill in any gaps. Here are a few such resources that might be helpful.

Cardano Time Handling

For more details, see the Time handling on Cardano section of https://docs.cardano.org.

In Cardano, the Ouroboros proof-of-stake (PoS) consensus protocol models the passage of physical time as an infinite sequence of time slots and epochs.

  • The actual time interval between blocks, or block time, is the slot length (in seconds) divided by the block coefficient f, which is the expected block frequency (blocks per second). For example, if f is 0.05, then on average 5% of slots contain blocks. If the slot length is 1 second, then the block time is 20 seconds.

  • An epoch is a period of time, containing some number of slots, used to select block-producing nodes. For example, in Shelley and later eras, an epoch consists of roughly 432,000 slots (or five days if we assume a slot length of 1 second).

  • The genesis block of Cardano was created on the 23rd of September 2017. As the first block in the blockchain, it set the foundation for the network, it does not reference any previous blocks, and it generated the initial supply of Ada.

  • A slot is a discrete time interval in which a single block may be produced; it is the fundamental time unit within the blockchain's consensus protocol. Slots should be long enough for a new block to have a good chance to reach the next slot leader in time. For example, the slot length in the Byron era was 20 seconds, while in Shelley and later eras it is 1 second. Not every slot results in a new block. Indeed, in any given slot, one or more block-producing nodes are nominated (probabilistically based on stake distribution) to be slot leaders and given the opportunity to produce a new block. For example, in Shelley and later eras, on average only 0.05 of slots will produce a block (resulting in 20-second intervals between blocks). Slot number may refer to a slot's position within the current epoch or it may mean the absolute slot count since the genesis block. The context should make clear which meaning is intended.

The parameter values mentioned in the examples above,

  • block time = 20 seconds,

  • slot length = 1 second,

  • block coefficient = 0.05,

  • slots/epoch = 432,000,

are unlikely to change in the short-term. However, the longer term plan is to replace the current Ouroboros protocol with Ouroboros Chronos, which addresses timekeeping challenges by providing the first high-resilience cryptographic time source based on blockchain technology (see the Ouroboros Chronos blog post).